One of commonly practiced methods for enhancing the modulus of elasticity of a polyester film is to enhance the degree of molecular orientation by improving the orientation method or the like. Another method is to reinforce the polyester film by finely dispersing a compound having a high modulus of elasticity such as liquid crystal polyester in a non-liquid crystal polyester as disclosed in JP-A-61-78862 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-B-7-37577 (The term "JP-B" as used herein means an "examined Japanese patent publication") and JP-A-57-25354.
However, if the degree of molecular orientation is excessively raised to enhance the modulus of elasticity of the polyester film, the film is apt to break during the orientation. Thus, the practical enhancement of the modulus of elasticity of a polyester film has been limited.
On the other hand, the foregoing method which comprises finely dispersing a liquid crystal polyester having a high modulus of elasticity in a non-liquid crystal polyester to reinforce the polyester film is apparently effective. However, a typical liquid crystal polyester has a bad compatibility with a non-liquid crystal polyester. Thus, voids can easily be formed around the liquid crystal polyester. In particular, the oriented product of a film comprising a large amount of a liquid crystal polyester incorporated therein has a large amount of voids formed therein, making it impossible to provide a polyester film having a high modulus of elasticity.
It was reported in JP-B-7-37577, etc. that if the liquid crystal polyester to be incorporated in the non-liquid crystal polyester is fibrous, it can provide remarkable enhancement of the modulus of elasticity. A molten liquid crystal polyester which has just been extruded from an extruder which gives a shear rate as high as not less than 200 s.sup.-1 often assumes a fibrous form. However, once given a shearing action at a rate as low as 10 s.sup.-1, e.g., by being passed through a filter, even a fibrous liquid crystal polyester which has just been extruded turns spherical. As a result, the enhancement of the modulus of elasticity cannot be recognized as expected.